Prior art mechanical pressure sensors used in industry to recognize a decay in pressure are usually in one of three forms, a Burdon tube, a corrugated diaphragm or diaphragm capsule, or a bellows. The Burdon gauge is one in which the pressure is indicated directly by an attached pointed and scale. The Burdon tube is elliptical or flattened in cross section and bent into a circular form. One end is soldered to a central block through which the fluid enters and the other end is sealed and coupled by a link to a pivoted quadrant with teeth meshing with those of a pinion on the pointer spindle. An increase of pressure within the tube tends to change its cross section from elliptical to circular and the tube consequently uncoils slightly, thus turning the pointer. The movement of the pointer may be detected electronically to record established pressures.
Another direct-reading gauge utilitzes as the elastic pressure-sensitive element a hollow sealed disc-shaped capsule made from corrugated metal diaphragms. Changes in pressure cause motion of the center of the capsule and this motion is amplified by a mechanical linkage to control movement of the pointer over a calibrated dial. In order to distinquish changes in pressure, two of these gauges are used, one to establish the initial pressure, the other to recognize the differential from initial pressure. The two gauges may be housed in one container.
Thirdly, diaphragms or bellows may be used to actuate valves or switches when a preset pressure is attained. Such actuators must provide sufficient movement to cause actuation, either by direct movement or mechanical linkage. While they are extremely accurate in recording the achievement of the required pressure, they are slow in recognizing the decay of pressure.
Each of these prior art industrial sensors have been designed specifically to respond to an increase in pressure and have a "dead band" between the magnitude of pressure sensed and the decay in pressure before recognizing the change in pressure. A significant defect is that they require calibration against known pressures measured by liquid column instruments, such as the manometer or barometer, and have no compensating ability to contend with test pressure fluctuations. In consequence their ability to detect leakage is dependent on precise pressure control, and the response is too slow to distinquish subtle changes in pressure at industrial production speed requirements.
Another method of detecting leakage is by the use of a double-rod end cylinder which acts as a pump to fill the vessel being tested, and brining an air-jet or other signalling device into contact with the piston rod opposite to the pump. The air jet is then held stationary and any leakage in the vessel being tested will cause the piston to move away from the jet, releasing it and causing a signal to be emitted. This system is used when intensification of pressure is required for the test procedure. It can only be used for hydrostatic testing at medium or high pressures and is extremely slow in operation.
The most common industrial technique for detecting leakage is to charge a vessel with pressurized air and immerse it in a liquid and observe whether bubbles are emitted from the vessel walls. This time consuming technique can present impediment to the throughput of a production process and the quality of the test is strongly dependent upon the skill of the operator.